Holography is an image-recording process distinct from other image-recording processes; both the phase and amplitude of a wavefront that intercept the recording medium are recorded.
In the production of holograms in general, an object to be imagewise recorded is irradiated with a first component split from a coherent radiation source (e.g., from a laser). Irradiation reflected from the object is directed toward an appropriately sensitized recording medium (e.g., recording media based on photopolymers, hardened dichromated gelatin, or silver halide). A beam of reflected coherent radiation is commonly termed an object beam. At the same time, a second component split from the coherent radiation source is directed to the recording medium, bypassing the object. A beam of such coherent radiation is commonly termed a reference beam. The interference pattern resultant of the interaction of the reference beam and the object beam impinging on the recording medium is latently recorded in the recording medium. When the photoexposed recording medium is processed (e.g., for development of the latent recordation) and subsequently appropriately illuminated and observed at an appropriate angle (i.e., generally an angle correspondent with the incident angle of the reference beam), the irradiation is diffracted by the interference pattern (cf., the hologram) to reconstruct the wavefront that originally reached the recording medium as reflected from the object.
Holograms that are formed by allowing reference and object beams to enter a recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in a photopolymeric recording medium forms fringes of material with varying refractive indices that are approximately normal to the plane of the recording medium. When the hologram is "played back" for viewing using transmitted radiation, these fringes refract the transmitted radiation to produce real and virtual images. The present invention is directed to the production of such transmission holograms, particularly, but without limitation, those of the so-called volume phase type.
Historically, despite their unique optical capabilities, volume phase holographic elements have proven difficult to manufacture because they require an exposure with coherent radiation of sufficient energy to record interference structures that extend throughout the thickness of a photosensitive recording medium. In conjunction with their energy requirements, as well as their comparatively precise and exacting process requirements, the production of original transmission holograms (volume-phase or otherwise) is oftentimes burdened with high manufacturing costs, inefficient production times and poor yields. In light of these concerns, the mass production of transmission holograms has been investigated. Several of such methods implement known principles of "contact copying" to produce multiple copies of an original master transmission hologram. (The principles of contact copying are discussed further below.)
Among the previously available automated systems for contact copying transmission holograms are "freeze-frame" type systems, cf., U.S. Pat. No. 4,209,250, issued to James et al. on Jun. 24, 1980; and U.S. Pat. No. 4,416,540, issued to Nicholson on Nov. 22, 1983. Such systems usually include components that advance recording materials, couple the materials to a master hologram, allow for settling time, and expose the film. The cycle time for "freeze-frame"-type systems is considered to be comparatively slow for very large volume production, and accordingly ill-suited for the goals sought and considered in the development of the present invention.
Apart from freeze-frame type systems, another system for contact copying transmission holograms is suggested in U.S. Pat. No. 4,973,113, issued to Harrington et al. on Nov. 27, 1990. Harrington et al. suggest the use of a "rotatable drum adapted to receive a beam of actinic radiation on a reflector in the drum reflecting a portion of the beam through a master to a recording medium to form the transmission hologram in the recording medium." The mirror used to reflect light in the embodiments suggested by Harrington, et al. is mounted directly to their rotatable drum; the mirror--as well as the direction of the light reflected thereby--moves correspondent with the rotation of the drum. By such configuration, the reflected light is continuously and blanketwise transmitted through a mounted master transmission hologram for a duration required to effect contact copying.
In consideration of the goals sought by the applicants, the implementation of Harrington, et al.'s apparatus is regarded to be constrained by several of its inherent aspects. First, the apparatus appears to require a large, flood-like exposure area, thereby compelling comparatively complicated control mechanisms for maintaining appropriate and desired exposing conditions. Second, it appears that unless a very high power laser is used to effect such overall exposure, a comparatively long exposure time would be required. During that time the holographic recording material and master transmission hologram could alter their relative positions and a hologram with unclear fringes would result. Third, since the mirror is disclosed as being mounted directly within and onto the drum, it is concluded that the mirror would be susceptible to vibrations resultant of drum rotation. Such vibrations may disturb the phase relationships necessary for effecting a desirable contact copy of the master transmission hologram. Further, the flood-like overall exposure involved with such apparatus together with the fixedly drum mounted mirror is believed to substantially foreclose the ability to control the angle of incidence used to effect reconstruction of a master transmission hologram onto a recording medium.
The limitations of the existing art highlight the need for an apparatus to mass produce transmission holograms more reliably, efficiently, and with greater flexibility.